The present invention relates to a crosslinking agent and a crosslinkable solid polymer electrolyte using the same. More particularly, the present invention relates to a crosslinking agent comprising methyl siloxane polymer of a backbone having polyalkylene oxide groups as branch groups and acryl groups being crosslinkable by heat or light irradiation at both ends thereof, and a crosslinkable solid polymer electrolyte composition comprising the crosslinking agent, a lithium salt, an organic solvent, and a curing initiator mixed at an appropriate ratio. The crosslinkable solid polymer electrolyte composition has a high ionic conductivity at room temperature and can be readily formed into a film and is thus suitable for use in large-size lithium-polymer secondary batteries applicable to electric cars, power storage devices for load leveling and the like, as well as in small-size lithium-polymer secondary batteries applicable to video cameras, portable data terminals such as cellular phones and notebook computers, and the like.
Electrochemical devices using solid electrolytes have been the subject of intense investigation and development due to the advantages that they can not only provide high charging/discharging efficiency, can be used in manufacturing various types of batteries, can be manufactured in film types thus being prepared in small-sized apparatuses but also they are leakage-free compared with electrochemical devices using liquid electrolytes. Especially, lithium-polymer batteries using polyalkylene oxide (PAO) based solid polymer electrolytes can be employed widely since they can provide batteries of high energy density.
The PAO based solid polymer electrolyte was discovered by P. V. Wright in 1975(British Polymer Journal, vol. 7, p. 319), and M. Armand named it xe2x80x9cion conducting polymerxe2x80x9d in 1978. Since that time, its use in the electrochemical devices has gradually increased. The typical solid polymer electrolyte is composed of polymers having electron-donating atoms such as oxygen, nitrogen and phosphorus, together with a complex of a lithium salt. The most typical example consists of polyethylene oxide (PEO) and a complex of a lithium salt. This shows a low ion conductivity of about 10xe2x88x928 S/cm at room temperature, and therefore, it cannot be applied to the electrochemical devices operated at room temperature. However, it can be used as a power source for electrochemical devices operated at a elevated temperature.
The ion conductivity of the solid polymer electrolyte usually increases as the segmental motion of the polymer chain increases. Therefore, the crystalline region within the polymer structure should be minimized to increase the amorphous region. In connection with this, Blonsky et al. reported on the application of poly(bis(methoxyethoxyethoxy)phosphazine) (J. Am. Chem. Soc., 106, 6854(1984)). Pantaloni et al. reported on the applicability of poly(ethoxyethoxyethoxy)vinylether (Electrochim. Acta, 34, 635(1989)). Further, there is disclosed a method for forming an interpenetrating polymeric network containing polyalkylene glycol modified to include terminal unsaturated hydrocarbon groups, a liquid electrolyte and an electrolyte salt by exposure to ultraviolet or electron beam radiation (U.S. Pat. No. 4,830,939; J. Electrochemm. Soc., 145, 1521(1998)). However, the branched or network solid polymer electrolytes showed an ion conductivity of about 10xe2x88x925-10xe2x88x924 S/cm at room temperature and poor mechanical properties when they were formed into films.
Accordingly, in order to solve these problems, a solid polymer electrolyte introducing low molecular weight polyethylene oxides to vinylidenfluoride-hexafluoropropene copolymers was disclosed to improve the ionic conductivity (Chem. Mater., 9, 1978, (1997)). Another solid polymer electrolyte using a crosslinking agent having three ethylene glycol acrylates at the center of cyclic alkyl, hetero alkyl molecule was also introduced to reinforce mechanical properties (KR Pat. Publication No. 01-4121 and KR Pat. Appl. No. 01-12913).
Studies on polyalkylene oxide backbones of crosslinkable and branched solid polymer electrolytes have been intensified because of expectation that polysiloxane polymer having polyethylene oxide side chains may move sufficiently due to its own flexibility and a low glass transition temperature (Macromol. Chem. Rapid Commun., 7(1986) 115, U.S. Pat. Nos. 4,673,718, 4,766,185, 5,227,043, and 5,440,011, Japanese Patent Laid-open No. 5-290616). However, these polysiloxane solid polymer electrolytes have poor mechanical properties and 10xe2x88x924 S/cm of an ionic conductivity at room temperature, and are thus not suitable for use in lithium batteries.
Accordingly, an object of the present invention is to provide a crosslinking agent to be used for solid polymer electrolyte comprising methyl siloxane polymer having polyalkylene oxide groups as a branch group and 2 to 4 acryl groups at both ends thereof.
Another object of the present invention is to provide a crosslinkable solid polymer electrolyte comprising the above crosslinking agent, a lithium salt, a plasticizer, and a curing initiator.
The solid polymer electrolyte having the crosslinking agent of the present invention exhibits enhanced mechanical properties and electrochemical stabilities due to its three dimensional network structure and can be readily formed into a film with high ionic conductivity at room temperature by facilitating the introduction of a plasticizer such as a polyalkylene oxide having low molecular weight and an organic solvent which can improve the conductivity. Thus, the solid polymer-electrolyte of the present invention is advantageously applied to lithium-polymer secondary batteries due to excellent adhesive property to electrodes and chemical and electrochemical stabilities.